Bandgap voltage reference circuits are well known. Such circuits provide for a summation of two voltages having opposite variations with temperature. The first voltage corresponds to a forward biased p-n junction having a Complimentary to Absolute Temperature (CTAT) variation. A first order temperature insensitive voltage is generated by adding a CTAT voltage to a Proportional to Absolute Temperature (PTAT) voltage such that the two slopes compensate each other. The PTAT voltage is generated by amplifying the base-emitter voltage difference of two transistors operating at different collector current density.
An example of such a low noise implementation of a bandgap voltage reference is described in FIG. 1. The bandgap voltage circuit of FIG. 1 consists of three pnp bipolar transistors, QP1, QP2, QP6, four npn bipolar transistors QN1, QN2, QN6, QN7, three resistors, R1, R2, R5, an amplifier, A, and a capacitor, C1. The emitter area of the bipolar transistors are: QN1, unity emitter area; QN2, n1 times unity emitter area; QP2 unity emitter area; QP1, n2 times unity emitter area; QP6, n3 times unity emitter area; QN6, n4 times unity emitter area; QN7, n5 times unity emitter area. The role of QP6, QN6 and QN7 is to reduce the collector and base current of QP1 and QN1 and by consequence to reduce the low band noise. The low band noise of the circuit of FIG. 1 is low as all transistors, except QP1 and QN1, are diode connected and QP1 and QN1 are operating with very low base current, due to the shunting sub-circuit of QP6, QN6, and QN7.
The nominal output voltage reference of the circuit of FIG. 1 is about 2.5V corresponding to two CTAT voltages (base-emitter voltages of QN1 and QP2) plus a balanced PTAT voltage (voltage drop across R2). For lower supply voltage (less than 2.5V) a lower nominal voltage will be preferred. For low cost it is also important to implement a bandgap voltage reference based on a single type bipolar transistor, preferable npn.